Comparative analysis of the expression and oncogenic activities of Xenopus c-, N-, and L-myc homologs

Isolation, characterization, and expression of proto-oncogene cMyc in large yellow croaker Larimichthys crocea

cMyc is a vital transcription factor that involves in the regulation of cell proliferation, growth, differentiation, and apoptosis. In the present study, cMyc in Larimichthys crocea (Lc-cMyc) was cloned and analyzed for investigating its function. The full-length cDNA of Lc-cMyc was 2089 bp encoding a 440-amino-acid protein (Lc-cMyc). Lc-cMyc had the characteristic helix-loop-helix-leucine-zipper (HLH-LZ) DNA-binding domain and highly conservative in evolution. The expression of Lc-cMyc was detected by quantitative real-time PCR (qRT-PCR) and in situ hybridization, respectively. In tissues, the gender differences of Lc-cMyc expression existed only in gonad and Lc-cMyc was extremely significantly expressed in ovary with the highest level in 635-dph ovary, especially in stages II (late) and III (early) oocytes. A certain degree of expression was examined in head kidney of both sexes and testis with high expression in spermatocyte. In embryos, Lc-cMyc was expressed at all embryonic stages. In early embryogenesis (from two-cell stage to mutiple-cell stage), Lc-cMyc was expressed very highly with a peak at two-cell stage. In late embryogenesis (from blastula stage to 1-day-post-hatching stage), the high expression of Lc-cMyc was detected as the following order: 1-day-post-hatching stage > pre-hatching stage > the appearance-of-optic-vesicles stage = mutiple-cell stage > beginning-of-heart-pulsation stage. The distribution of Lc-cMyc concentrated gradually in the back of embryos until in the head. In conclusion, the spatio-temporal expression patterns of Lc-cMyc indicated an essential role in oogenesis and embryogenesis and contributed to insight into the molecular mechanisms of regulating pluripotency in large yellow croaker.

c-myc in whitefish (Coregonus lavaretus): structure, expression, and insights into possible posttranscriptional regulatory mechanism

c-myc has a crucial function in growth control, differentiation, and apoptosis of vertebrate cells. Despite the important role of c-myc in mediating the biological effects, studies of c-myc gene expression and factors that control it in organisms other than mammals, such as fish, have been rare. In the current study, we asked whether c-myc mRNA of whitefish, a feasible organism for pollution monitoring in aquatic systems and a model in toxicological research, contains activity sites for regulatory motifs in its 5'- and 3'-UTRs, similar to those found in mammals. We were particularly interested in whether miRNA-34, a known negative regulator of c-myc's in mammals, is able to regulate c-myc in fish. To answer these questions, we determined the mRNA sequence of whitefish c-myc and inferred the structure of the protein that it codes for. We found that the active sites of mRNA and structures of the inferred c-myc protein are similar to those found in mammals and other fish. Remarkably, levels of c-myc mRNA expression were very high in ovaries compared to other tissues of whitefish, thus corroborating previous data in fish. Using bioinformatic searches on c-myc 3'-UTR, we confirmed the presence of two miRNA-34a (miR-34a) response elements. Luciferase reporter assay showed that activity of reporters containing either the miR response elements or entire c-myc 3'-UTR was significantly reduced (p < 0.001) by ectopic expression of miR-34a. Therefore, we further investigated possible involvement of miR-34a in c-myc gene silencing by profiling the expression of both genes in livers of whitefish treated for 8, 24, 48 h with MC-LR, a potent c-myc inducer in mammals. Although the difference was only significant at p = 0.08, the expression of c-myc mRNA in challenged whitefish after 24 h of the treatment was notably higher than that in livers of control fish. Concurrently, we noticed slight but significant up-regulation of miR-34a after 24 and 48 h of the challenge (p < 0.05); however, we found no significant correlation of the c-myc mRNA levels and miR-34a expression. Together, these results suggest that miR-34a might regulate c-myc gene expression in whitefish liver; however, their involvement in MC-LR hepatotoxicity should be clarified in future studies.

The myc road to hearing restoration

Current treatments for hearing loss, the most common neurosensory disorder, do not restore perfect hearing. Regeneration of lost organ of Corti hair cells through forced cell cycle re-entry of supporting cells or through manipulation of stem cells, both avenues towards a permanent cure, require a more complete understanding of normal inner ear development, specifically the balance of proliferation and differentiation required to form and to maintain hair cells. Direct successful alterations to the cell cycle result in cell death whereas regulation of upstream genes is insufficient to permanently alter cell cycle dynamics. The Myc gene family is uniquely situated to synergize upstream pathways into downstream cell cycle control. There are three Mycs that are embedded within the Myc/Max/Mad network to regulate proliferation. The function of the two ear expressed Mycs, N-Myc and L-Myc were unknown less than two years ago and their therapeutic potentials remain speculative. In this review, we discuss the roles the Mycs play in the body and what led us to choose them to be our candidate gene for inner ear therapies. We will summarize the recently published work describing the early and late effects of N-Myc and L-Myc on hair cell formation and maintenance. Lastly, we detail the translational significance of our findings and what future work must be performed to make the ultimate hearing aid: the regeneration of the organ of Corti.

Evolutionary history of c-myc in teleosts and characterization of the duplicated c-myca genes in goldfish embryos

c-Myc plays an important role during embryogenesis in mammals, but little is known about its function during embryonic development in teleosts. In addition, the evolutionary history of c-myc gene in teleosts remains unclear, and depending on the species, a variable number of gene duplicates exist in teleosts. To gain new insight into c-myc genes in teleosts, the present study was designed to clarify the evolutionary history of c-myc gene(s) in teleosts and to subsequently characterize DNA methylation and early embryonic expression patterns in a cyprinid fish. Our results show that a duplication of c-myc gene occurred before or around the teleost radiation, as a result of the teleost-specific whole genome duplication giving rise to c-myca and c-mycb in teleosts and was followed by a loss of the c-mycb gene in the Gasterosteiforms and Tetraodontiforms. Our data also demonstrate that both c-myc genes previously identified in carp and goldfish are co-orthologs of the zebrafish c-myca. These results indicate the presence of additional c-myca duplication in Cyprininae. We were able to identify differences between the expression patterns of the two goldfish c-myca genes in oocytes and early embryos. These differences suggest a partial sub-functionalization of c-myca genes after duplication. Despite differences in transcription patterns, both of the c-myca genes displayed similar DNA methylation patterns during early development and in gametes. Together, our results clarify the evolutionary history of the c-myc gene in teleosts and provide new insight into the involvement of c-myc in early embryonic development in cyprinids.

Nucleosome regulator Xhmgb3 is required for cell proliferation of the eye and brain as a downstream target of Xenopus rax/Rx1

Rax/Rx is a paired-type homeodomain-containing transcription factor that is essential for cell proliferation in the developing eye and brain. The molecular mechanisms that regulate cell proliferation by rax, however, are largely unknown. Here, we identify the high mobility group B3 gene (hmgb3) as a downstream target of Xenopus rax (Xrax/XRx1). Overexpression of Xhmgb3 results in an increase in eye and brain sizes due to promoted cell proliferation, while morpholino-oligo-mediated knock down of Xhmgb3 reduces eye and brain sizes. In addition, ChIP assays showed that Xhmgb3 is recruited around the promoter region of c-myc to enhance c-myc transcription. We also found that XOptx2 requires rax for its initial expression. Furthermore, we show that Xhmgb3 and XOptx2 are required for retinal development mainly at different developmental stages. Our findings reveal a novel aspect of progenitor cell proliferation during embryonic central nervous system (CNS) development.

Isolation and comparative expression analysis of the Myc-regulatory proteins Mad1, Mad3, and Mnt duringXenopus development

The Myc-Max-Mad network of transcription factors plays an essential role in many cellular processes such as proliferation, differentiation, and apoptosis. The Mad proteins heterodimerize with Max, function as transcriptional repressors, and are capable of antagonizing the transforming activity of Myc. We report on the isolation of Xmad1, Xmad3, and Xmnt, novel Xenopus genes belonging to the Mad family. We also describe their temporal and spatial expression patterns during Xenopus embryogenesis. Xmad1 expression is found primarily in cells that have undergone terminal differentiation including the notochord, floor plate, and cement gland. Xmad3 transcripts are expressed broadly throughout the central nervous system and the eye, starting at neurula stages. In contrast, Xmnt expression in the CNS was localized anteriorly and, in addition, is present in the migrating neural crest cells. This study demonstrates the Mads are expressed in specific and mostly nonoverlapping patterns, suggesting distinct roles during embryogenesis.

The protooncogene c-myc is an essential regulator of neural crest formation in xenopus

The neural crest, a population of multipotent progenitor cells, is a defining feature of vertebrate embryos. Neural crest precursor cells arise at the neural plate border in response to inductive signals, but much remains to be learned about the molecular mechanisms underlying their induction. Here we show that the protooncogene c-Myc is an essential early regulator of neural crest cell formation in Xenopus. c-myc is localized at the neural plate border prior to the expression of early neural crest markers, such as slug. A morpholino-mediated "knockdown" of c-Myc protein results in the absence of neural crest precursor cells and a resultant loss of neural crest derivatives. These effects are not dependent upon changes in cell proliferation or cell death. Instead, our findings reveal an important and unexpected role for c-Myc in the specification of cell fates in the early ectoderm.

Amplification and overexpression of the L-MYC proto-oncogene in ovarian carcinomas

Gene amplification is an important mechanism of oncogene activation in various human cancers, including ovarian carcinomas (OvCas). We used restriction landmark genomic scanning (RLGS) to detect amplified DNA fragments in the genomes of 47 primary OvCas. Visual analysis of the RLGS gel images revealed several OvCa samples with spots of greater intensity than corresponding spots from normal tissues, indicating possible DNA amplification in specific tumors. Two primary tumors (E1 and S12) shared four high-intensity spots. A recently developed informatics tool termed Virtual Genome Scans was used to compare the RLGS patterns in these tumors with patterns predicted from the human genome sequence. Virtual Genome Scans determined that three of the four fragments localized to chromosome 1p34-35, a region containing the proto-oncogene L-MYC. Sixty-eight primary OvCas, including 40 analyzed by RLGS, were screened by quantitative polymerase chain reaction (PCR) for possible amplification of L-MYC. Ten tumors with increased L-MYC copy number were identified, including tumor E1, which showed an approximately 24-fold increase in copy number compared to normal DNA. Southern analysis of several tumors confirmed the quantitative PCR results. Using sequence tagged site (STS) markers flanking L-MYC, increased DNA copy number in tumor E1 was found to span the region flanking L-MYC between D1S432 and D1S463 ( approximately 3.1 Mb). Other tumors showed amplification only at the L-MYC locus. Using oligonucleotide microarrays, L-MYC was found to be more frequently overexpressed in OvCas than either c-MYC or N-MYC relative to ovarian surface epithelium. Quantitative reverse transcriptase-PCR analysis confirmed elevated L-MYC expression in a substantial fraction of OvCas, including nine of nine tumors with increased L-MYC copy number. The data implicate L-MYC gene amplification and/or overexpression in human OvCa pathogenesis.

Differential expression of max and two types of c- myc genes in a tetraploid fish, the common carp ( Cyprinus carpio )

We cloned the full-length cDNA of max gene from the common carp (Cyprinus carpio). The cDNA clone of carp max consists of 1209 bp and contained an ATG-initiated ORF consisting of 156 aa. The carp MAX share 76.7-93.8% aa identity with those of human, mouse, rat, chicken, Xenopus and zebrafish, respectively. The 15 bp alternative splicing was observed in the loop region of helix-loop-helix and is not previously described in mammalian max sequences. Transcripts of max gene were observed in all of the tissues of carp investigated in this study. The highest expression was found in the ovary, and the transcripts in hepatopancreas and heart were low. Two carp c-myc genes (CAM1 and CAM2) showed differential expression pattern. The expression of max was concomitant with CAM2 expression, but not with CAM1. It has been reported that MYC/MAX heterodimer as a regulator of gene expression has been maintained throughout vertebrate evolution, and the expression of c-myc has been concomitant with max expression. In addition, according to phylogenetic analysis, CAM1 is evolving faster than CAM2 after gene duplication. Therefore, this result suggests that CAM1 may evolve to obtain a new function different from c-myc.

Expression of a down-regulated target, SSeCKS, reverses v-Jun-induced transformation of 10T1/2 murine fibroblasts

Line 10T1/2 mouse fibroblast overexpressing the v-Jun oncoprotein were morphologically altered, grew into multilayered foci in culture and formed colonies when suspended in agar. The growth rate of the v-Jun-transformed 10T1/2 cells was not changed significantly from that of the untransformed parental cells, but the saturation density of the transformed cultures exceeded that of normal controls by a factor of 2. mRNA extracted from v-Jun-transformed 10T1/2 cells was analysed for differential gene expression with DNA micro-array technology. One of the targets downregulated by v-Jun was identified as SSeCKS (Src-suppressed C kinase substrate). Re-expression of SSeCKS in v-Jun-transformed fibroblasts reversed the transformed phenotype of the cells. Their ability to form foci was reduced to background levels, the number and size of agar colonies was lowered by a factor of 10 and the saturation density was significantly diminished. However, expression of SSeCKS had little effect on the morphology of v-Jun-transformed 10T1/2 cells. These data suggest that the SSeCKS protein has growth-attenuating properties. Down-regulation of SSeCKS may be essential for Jun-induced transformation.

N-myc can functionally replace c-myc in murine development, cellular growth, and differentiation

Members of the myc family of cellular oncogenes have been implicated as transcriptional regulators in pathways that govern cellular proliferation and death. In addition, N-myc and c-myc are essential for completion of murine embryonic development. However, the basis for the evolutionary conservation of myc gene family has remained unclear. To elucidate this issue, we have generated mice in which the endogenous c-myc coding sequences have been replaced with N-myc coding sequences. Strikingly, mice homozygous for this replacement mutation can survive into adulthood and reproduce. Moreover, when expressed from the c-myc locus, N-myc is similarly regulated and functionally complementary to c-myc in the context of various cellular growth and differentiation processes. Therefore, the myc gene family must have evolved, to a large extent, to facilitate differential patterns of expression.

Determination of heterogeneous transcription start points of two c-myc genes from the common carp (Cyprinus carpio)

We determined the heterogeneous transcription start points (tsp) of two c-myc genes from the common carp (Cyprinus carpio), tetraploid teleost, by the oligo-capping method and showed the existence of the first exon. This is the first report on the existence of the first exons of the fish c-myc gene. Transcription of the two carp c-myc genes started from at least four sites in CAM1, locating from -752 to -381bp upstream of the translation start site, and from 12 sites in CAM2, locating from -586 to -413bp upstream respectively. The first introns of CAM1 and CAM2 were deduced to be 335 and 356bp, respectively. They shared 86.9% nt identity, lower than those of the second exons (94.1%), and third exons (92.3%), which suggest that the first exons evolved faster. No nt identities were found between the c-myc first exons of carp and other vertebrates. The putative promoter regions in CAM1 and CAM2 contained no obvious TATA or CCAAT boxes in the expected positions.

Alternative splicing and embryonic expression of the Xenopus mad4 bHLH gene

The cell proliferative activity of the Myc family of basic helix-loop-helix/leucine zipper (bHLHZip) transcription factors is dependent upon binding to the ubiquitous Max protein. In the absence of heterodimerization with Max, Myc protein is unable to efficiently bind to DNA and activate transcription. Members of the Mad family of transcription factors are thought to modulate the cell proliferative effects of the c-myc proto-oncogene by binding to Max, directly competing with the Myc protein for both heterodimerization and DNA binding. Consistent with a role in down-regulating cell division, the murine mad genes are expressed in embryonic tissues undergoing differentiation, often during or shortly after the down-regulation of myc gene expression. Here, we report the isolation and characterization of the first Xenopus mad family member, Xmad4. Maternal Xmad4 transcripts are present at high levels in the oocyte and in the cleavage stage embryo, but almost disappear by the neurula stage. Zygotic expression of the Xmad4 gene is initiated in the epidermis of the late neurula stage, and shortly thereafter, Xmad4 is transiently detectable in the cement and hatching glands. At later stages, expression is also observed in the developing pronephros and liver. Unlike the murine mad4 gene, we find that multiple Xmad4 splice variants exist in Xenopus and that these variants are differentially expressed in both the embryo and the adult. Despite the demonstrated antagonistic role of Mad proteins in the regulation of Myc activity, we show that the over-expression of Xmad4 in the cleavage-stage embryo has no detectable phenotypic effect, suggesting that Myc function is dispensable during early embryonic development.

The amplification of oligonucleotide themes in the evolution of the myc protooncogene family

The evolutionary past of intragenic repeats in protein-coding exons of c-, N-, L-, and s-myc-protooncogene subfamilies was elucidated. Apparently these genes evolved by succession of distinct unit events rather than by a steady flow of random point mutations. An evolutionary event probably involved a duplication of the whole gene, which was followed by amplification of progressively shorter oligonucleotide themes and motifs. The repeats were either joined in tandem or one of the copies was transposed and integrated elsewhere within the same exon. In some instances multiple fragments of an amplified theme were integrated at several sites. Direct repeats were found to prevail over inverted ones. By reconstructing the fate of repeats in the course of evolution of vertebrates, the origins of some functional domains could be traced to the initial amplification event. For example, an N-myc-specific domain was created by tandem duplication of a single-copy theme of L-myc exon; at the time of divergence of the c-myc and N-myc, the tandem duplex underwent a new round of duplication followed by transposition of the new copy, thus accounting for the formation of a new domain specific for c-myc. The model proposed here may be regarded as a molecular-level equivalent of the theory of punctuated equilibria.

An amino-terminal domain of Mxi1 mediates anti-Myc oncogenic activity and interacts with a homolog of the yeast transcriptional repressor SIN3

Documented interactions among members of the Myc superfamily support a yin-yang model for the regulation of Myc-responsive genes in which transactivation-competent Myc-Max heterodimers are opposed by repressive Mxi1-Max or Mad-Max complexes. Analysis of mouse mxi1 has led to the identification of two mxi1 transcript forms possessing open reading frames that differ in their capacity to encode a short amino-terminal alpha-helical domain. The presence of this segment dramatically augments the suppressive potential of Mxi1 and allows for association with a mammalian protein that is structurally homologous to the yeast transcriptional repressor SIN3. These findings provide a mechanistic basis for the antagonistic actions of Mxi1 on Myc activity that appears to be mediated in part through the recruitment of a putative transcriptional repressor.

Inhibition of ras-induced proliferation and cellular transformation by p16INK4

The cyclin-dependent kinase 4 (CDK4) regulates progression through the G1 phase of the cell cycle. The activity of CDK4 is controlled by the opposing effects of the D-type cyclin, an activating subunit, and p16INK4, an inhibitory subunit. Ectopic expression of p16INK4 blocked entry into S phase of the cell cycle induced by oncogenic Ha-Ras, and this block was relieved by coexpression of a catalytically inactive CDK4 mutant. Expression of p16INK4 suppressed cellular transformation of primary rat embryo fibroblasts by oncogenic Ha-Ras and Myc, but not by Ha-Ras and E1a. Together, these observations provide direct evidence that p16INK4 can inhibit cell growth.